A band filter is a device that is able to separate an incoming optical spectrum into bands (i.e., when used as a demultiplexer) or combine separate optical bands into an output optical spectrum (i.e., when used as a multiplexer), ideally with sharp passband corners. Band filters are useful in both dense (D) and coarse (C) wavelength-division multiplexed (WDM) systems. In DWDM systems, band filters are typically used for de/multiplexing with a high spectral efficiency or for permitting the use of narrowband optical amplifiers, dispersion compensators, add-drop filters, etc. In CWDM systems, band filters are typically used for de/multiplexing without transmitter temperature control.
In current optical systems, a substantial portion of the implemented band filters are based on thin-film interference filters. Thin-film interference filters typically have very low losses however, they require expensive hermetic packages and tedious hand assembly. Thin-film interference filters also typically exhibit significant chromatic dispersion, they require more than ten cavities to achieve sharp passband corners, and they typically require a cascade to demultiplex more than two bands.
Band filters based on bulk gratings have also been proposed. This type of band filters can be dispersion-free and can de/multiplex more than two bands without a cascade, however, they exhibit more loss than their thin-film counterparts. Bulk grating band filters also comprise large form factors, and, like thin-film filters, require expensive hermetic packages and tedious hand assembly.
Finally, band filters based on planar lightwave circuits (PLCs) have also been proposed. Silica waveguide PLCs do not require hermetic packaging and they typically do not require optical alignment (except for an automatable fiber attachment). PLC band filters are capable of being dispersion-free, and they can readily have sharp passband corners. However, the previously proposed band filters based on PLCs (also known as MZI-based filters) become exceedingly large in form factor and are very demanding to de/multiplex more than four bands and/or to achieve sharp passband corners.